beneficiation plant

This paper examine the optimization of a beneficiation plant for recovering marketable barites from crude ores of different characteristics. The plant, part of a mining complex in Sardinia, consists of a jig section integrated with a flotation line. The problem has been studied using a suitable model of the process, based upon experimental and field data pertaining to both the characteristics of the feed material and the performance of machinery. Data have been automatically processed with the aid of a computer to find the optimum setting of each section, in order to maximize profit for each kind of ore separately fed to the plant. Moreover, the advantages of blending the various concentrates — not all of them individually marketable — are also demonstrated.

The overall economic result can be improved provided suitable proportions of each kind of ore are fed to the plant. Such information is fundamental for the long-term planning of exploitation of available reserves, and provides guidelines for further prospecting.

The paper illustrates the washability characteristics of the ores and describes in detail the model adopted. The results of data processing are then presented in the form of computer graphs and discussed. Finally, conclusions are drawn regarding the advantages of resorting to Operations Research as an aid to management.

Barite Introduction

At the Barega mine, in Sardinia, ore reserves are contained in a number of orebodies of varying characteristics. They are mined either opencast or underground, and the resulting run-of-mine can be distinguished as follows, according to BaSC>4 grade and intergrowLh features:

  • a crude ore easily washable with gravity methods yielding relatively good recoveries (ore A, from Litopone stope);
  • a crude ore of lower grade containing finely disseminated quart2 which yields a poor quality concentrate at the expense of considerable losses in the coarse waste (ore B, from Gianni stope).

The plant consists of three main sections:

  • a three-stage crushing station where the R.O.M. is reduced from a top size of about 600 mm down to – 20 mm;
  • a two-stage jigging section with interstage grinding whereby a gravity concentrate is produced;
  • a flotation line for the recovery of ba— rite values still contained in the trine fractions discarded from the preceuing section.

Each kind of ore is stocked in different stockpiles and can be fed separately to the plant. Crushing is done with the same machine setting, irrespective of the ore being processed.

The reject of the screen-controlled cone crusher is collected in a 1200 tonnes bin at the head of the upgrading plant.

Gravity treatment is carried out in two stages. In the first stage the ore is preconcentrated in two parallel lines, each consisting of a vibrating bed jig with a capacity of 60 L/h. The coarse waste is separated, screened and sold as an aggregate for concrete. The finest size class ( -3 mm) may be suitable for flotation, provided it contains enough recoverable barite.

The preconcentrate and the fines filtered through the jig bed are dewatered in a spiral classifier: the overflow is sent to a Dorr thickener whereas the coarse fraction is screened at 8 mm, which is the liberation size for the final gravity concentration. A short-head cone crusher reduces the screen reject to below that size. The crushed preconcentrate is split and stored in four bins, each feeding a stationary-bed jig with a capacity of 7 t/h for final cleaning. Both concentrate and middlings are dewatered; overflow fines are thickened and sent to flotation together with preconcentration fines while middlings follow the same route as the preconcentration waste.

Settled pulp from v.ho Dorr thi cko-.-ir-r is fed to the flotation line. The material is first cycloned; this operation generally produces a 10 to 15 BaS04 points increase in the underflow grade with tolerable barite losses. This underflow is ground to – 0.5 mm in a ball mill in closed circuit with a rake classifier; after conditioning with Na-Cethylsulfate, the reject is processed via flotation with two cleaning stages obtaining a commercial filter cake assaying 94 – 95% BaSO,, suitable for barium chemicals.

The optimization problem

The barite market is today undergoing a worldwide slump. In fact, owing to the recent fall in oil prices, the demand for drilling muds applications, by far the largest outlet for barite concentrates,has suddenly diminished.

Consequently, the market structure has undergone a major change compared to the previous period. This chiefly concerns the more stringent quality requirements for the different utilizations: oil service companies, in addition to OCMA (Oil Companies Materials Association) standards, tend now to refuse blends with products containing flotation reagents, whereas manufacturers of barium salts accept only high-grade concentrates with very low pollutants. Moreover, over the last few years market prices have fallen considerably in real terms.

Under these circumstances, the production schedule had to he adjusted; presently, the plant output consists of the following products :

  • a high-grade, low silica gravity concentrate for barium chemicals;
  • a 4.20 S.G. jig concentrate suitable as weighing agent for drilling muds (after grinding) or heavy concrete;
  • a flotation filter cake for barium chemicals;

Therefore the main problem is how to optimize the production schedule in order to achieve the maximum profit while satisfying market demand.

As regards the ore, the basic data to be included in the process model are the washa-bility characteristics and the proportion of each kind of ore being beneficiated. This proportion must be consistent with the level of known reserves available at the mine. Washability characteristics have been determined by sink-float analysis using heavy liquids or FeSi suspensions in TBE (tetra-broinoethane) for the higher densities.

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